Thursday, May 9, 2013


There ain't no such thing as carbon neutrality.

Or, at least, exceptions to that rule are few and far between.

Besides scarcity, greenhouse gas emissions are the second major concern with our current sources of energy.  But one of the most frequent fallacies I encounter is the contention that if any alternative source also produces a greenhouse gas at all, or does not have a negative carbon balance,  it is somehow not "green" or is just as bad as using fossil fuels.

To be fair, it's true that most so-called "green" methods of energy production will also produce, on balance, some amount of carbon emissions beyond what they "save."  On an absolute basis, if you were to track, say, biomass co-firing in a power plant, you would find that in addition to literally burning all of the carbon you sequestered, you required additional carbon emissions from harvesting, transport, pelletization and other processing, and caused slightly more carbon dioxide to be emitted from the plant by reducing its power conversion efficiency.  The carbon balance is positive - biomass co-firing on net emits carbon dioxide.

But this is true of basically every source of energy.  While it's trivially true for fossil fuels, taking a survey of every "green" energy source comes up with almost the same result. Solar thermal and PV have fabrication costs that emit carbon dioxide, and even carbon capture and sequestration (which hasn't even been implemented) or natural geothermal is at best neutral, probably less than neutral if you account for emissions during fabrication and construction. None of these technologies actively remove carbon dioxide from the air while producing energy, so they're not carbon neutral.   Even Carbon Recycling International (CRI), of Iceland, which uses virtually emission-free hydroelectricity to make methanol from captured atmospheric CO2, isn't carbon neutral unless it uses its methanol for some non-decomposing chemical use.  If renewable methanol is burned for fuel, at best it's carbon neutral.

In short, with apologies to Heinlein, TANSTACN.

So why do we consider green technologies "green"?  Among other things, most of the time people operate under the assumption that a marginal unit of sustainable energy displaces a unit of fossil fuel.  Using that framework, a lifecycle analysis is usually able to claim a net benefit from its project, even if on net the amount of emissions is positive.  The point there is to replace something that is highly carbon intensive - a fossil fuel - with something that is much less carbon intensive.  

In part, this explains my generally sanguine attitude towards land use change from biomass energy.  Yes, it occurs, and when it occurs in places with rich biodiversity, vulnerable ecosystems or places where energy plantations will release more carbon from clearance than will ever be recovered then it's of course bad.  But when you talk about a type of land use change from biomass energy with low impact, such as the conversion of small amounts tallgrass prairie to high-density energy crop plantings*, I shrug my shoulders and ask, "So, would you rather be burning a fossil fuel?"

Once there isn't any fossil carbon energy left to displace and marginal energy inputs will have to come from sources like biomass, then things like local opportunity cost start becoming more significant.  When we live in a Paolo Bacigalupi-esque biomass economy, we will have to care about marginal carbon emissions from biomass.  But while there is fossil fuel being displaced, there will be a net benefit from sustainable energy.

* Not a random choice, by the way.  Mother Jones' climateprogress has a habit of moaning over small fry like this.

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